1. Field of the Invention
The present invention relates to the preparation of a coil-comb block copolymer and a method for producing nanostructures formed by the copolymer; and, more particularly, to a method for producing nanostructured polymer thin films, including: preparing a coil-comb block copolymer via a controlled polymer polymerization process; forming a thin film of the block copolymer on substrates made of various materials and carrying out heat treatment to form nanostructures including vertically aligned cylindrical microstructures; and irradiating ultraviolet rays to the thin film and carrying out oxygen plasma treatment to form nanostructured polymer thin films including cylindrical pores.
2. Description of Related Art
Block copolymers refer to polymers having a structure including two or more types of polymer blocks linked to each other. When chemically different polymer blocks are linked to each other, a so-called microphase separation phenomenon occurs, wherein each block forms a different microstructure depending on the length of the polymer block. Since general polymer blocks have a length of several tens nanometers, such microphase separation in block copolymers may be utilized as a method for producing nanostructures having various structures.
Particularly, since block copolymers aligned with cylindrical microstructures easily removable along the vertical direction to the substrate in a thin film state can be converted with ease into a porous polymer thin film having a high aspect ratio, they have received great attention. Possible applications of such block copolymers may include block copolymer lithography, separators incorporated into nanofiltration systems, molds for fabricating inorganic nanostructures, or the like.
Polystyrene-block-poly(methyl methacrylate) forms a cylindrical microstructure of the poly(methyl methacrylate) blocks surrounded with the polystyrene blocks in a thin film state, when the volumetric proportion of the poly(methyl methacrylate) block is approximately 0.3. When the thin film is irradiated with ultraviolet rays, the polystyrene blocks are crosslinked, while the poly(methyl methacrylate) blocks are decomposed. The thin film may be converted with ease into a porous polymer thin film by removing all of the poly(methyl methacrylate) blocks via additional oxygen plasma treatment.
However, in general, the cylindrical microstructures formed by the poly(methyl methacrylate) blocks are aligned in parallel with the substrate, because the polymer blocks showing higher interaction with the substrate cover the surface selectively before the cylindrical microstructures are formed. In the case of the most widely used silicon wafers, polystyrene blocks cover the surface before the cylindrical microstructure of poly(methyl methacrylate) blocks are aligned in parallel with the substrate on the polystyrene block layer.
Therefore, many attempts have been made to align the cylindrical microstructures vertically to the substrate. Typical examples of such attempts for aligning the nanostructures of a block copolymer thin film that have been developed to date include the methods as described hereinafter.
The neutral brush method includes preparing a random copolymer of polystyrene-poly(methyl methacrylate) having a hydroxyl group at one end thereof with a ratio of 0.64-0.36, and linking the random copolymer to a hydroxyl group-containing substrate via covalent bonding. Then, polystyrene-block-poly(methyl methacrylate) is spin-coated onto the substrate treated as mentioned above, followed by heat treatment at a temperature of 150° C. or higher, so as to obtain a vertically aligned cylindrical microstructure stably. As another neutral brush, random copolymers having a hydroxyl group in a part of the backbone, or those crosslinkable under heating or UV irradiation have been developed and used (Science 1997, 275, 1458-1460). However, the neutral brush method has limitations in that it additionally requires precise polymer synthesis, and it uses substrate surface having hydroxyl groups. Moreover, since the neutral brush still remains after forming the porous polymer thin film, it is not possible to obtain a pure substrate surface.
In addition, the electric field method uses the fact that each microstructure of a block copolymer has a different dielectric constant, and includes aligning nanostructures depending on the direction of an electric field upon the application of the electric field (Adv. Mater. 2000, 12, 787-791, International Patent Publication No. WO2001070873). However, the electric field method is disadvantageous in that it requires electrodes capable of applying an electric field to both surface of the block copolymer, it uses a strong electric field, and it is not applicable to thin films having a thickness greater than a certain thickness.
In addition, the graphoepitaxy method includes forming a pattern having irregularities on the surface of a substrate using, for example, UV lithography, and further forming a thin film of block copolymer thereon, so that the alignment of the block copolymer is determined by the direction of the pattern, when the size of the pattern used as a substrate is an integer multiple of the size of the nanostructures of the block copolymer (Science 2008, 321, 939-943, International Patent Publication No. WO200891741). However, the graphoepitaxy method requires forming an additional pattern on the surface of the substrate. Therefore, it is not possible to obtain nanostructures of pure block copolymer.
Further, the epitaxial self-assembly method includes forming a chemical pattern of an organic single molecular layer on the surface of a substrate to the same size as the nanostructures of a block copolymer, and further forming a thin film of block copolymer thereon, so that the nanostructures of the block copolymer are aligned vertically while the nanostructures of the block copolymer are formed along the chemical pattern of the surface of the substrate (Science 2005, 308, 1442-1446, International Patent Publication No. WO2006112887). However, the epitaxial self-assembly method is disadvantageous in that it uses expensive systems, such as extreme ultraviolet (EUV) or e-beam lithography systems, for a long time to form the chemical pattern of the organic single molecular layer. Moreover, the substrates that may be used in this method are limited to those capable of forming organic single molecular layers thereon.
Finally, Korean Patent Registration No. 10-08566 discloses a method including forming an oxide thin film on the surface of a substrate, and forming a neutral brush thereon to increase the diversity of the substrate. However, the method still has problems in that it additionally requires vacuum deposition of an oxide thin film and uses a neutral brush.
As a result, there is an imminent need for developing a method for overcoming the problems of the known methods using block copolymer thin films to form vertically aligned nanostructures and for producing a block copolymer thin film including a vertically aligned cylindrical microstructure on various kinds of substrates with ease. To satisfy such a need, it is required to develop a method for aligning the cylindrical microstructures in the vertical direction without surface modification of a substrate or without application of an electric field, etc.